Spectral sensitization of silver halide in photographic compositions is generally accomplished by adsorption of sensitizing dye molecules to the surface of silver halide grains. The dye molecules absorb energy of a certain wavelength and transfer that energy to the silver halide, causing formation of a latent image. For many applications, it is highly desirable that the dye that is adsorbed to the surface of the silver halide be in a J-aggregated state. This J-aggregrated state usually imparts to the dyes desirable sharp, intense absorbance, and also increases the efficiency with which the dyes absorb light and spectrally sensitize the silver halide, relative to non-aggregated dyes.
The situation generally regarded as providing maximum sensitization is coverage of the sensitizable surface of the silver halide grain with a layer of sensitizing dye the thickness of a single molecule, the so-called monolayer coverage. If this limit is exceeded and more dye is loaded onto the silver halide grain, creating a multimolecular-thick layer of the same sensitizing dye, photographic speed losses occur. These speed losses may arise from competing light absorption, which may occur when the outer dye molecules absorb energy from light of the wavelength to which it is desired to sensitize the silver halide, but due to their distance from the grain and the presence of other dye molecules interspaced between the outer dye molecules and the surface of the silver halide grain, they do not transfer energy to create the latent image. The dye molecules directly adsorbed to the surface of the silver halide, while capable of transferring energy to the silver halide, are not able to absorb all of the light energy due to this competition. Another common limitation of the amount of dye that may be adsorbed to silver halide emulsion grains is due to dye-induced desensitization. Desensitization occurs by a mechanism where the adsorbed dye traps photoholes and then releases the holes, which attack latent image centers. In actual practice, the onset of desensitization occurs when the dye coverage exceeds about 30% monolayer coverage.
It has been speculated that attaching sensitizing dye molecules to, or incorporating them in a polymer backbone could allow for amounts of sensitizing dye greater than that representing monolayer coverage to be loaded onto silver halide grains. Theoretically, higher spectral sensitivity from increased light absorption could be obtained utilizing a dye polymer than with adsorbtion of individual dye molecules onto the surface of the silver halide grain.
A significant problem with previous attempts to utilize polymeric sensitizing dyes has been their inability to attain the desireable state of J-aggregation when adsorbed to silver halide. While the exact cause of this inability to J-aggregate is not known, the discovery of the present invention indicates that it may be due to the lack of proper orientation of the dye molecule units to each other in the polymer molecule or to an insufficient proportion of dye molecule units relative to the polymer repeat unts, resulting in insufficient overlap of .pi. orbitals of the dye molecules to achieve J-aggregation.
U.S. Pat. No. 2,393,351 discloses polymeric sensitizing dyes for photographic emulsions containing two dye nuclei linked through heterocyclic nitrogen atoms by a polyvalent hydrocarbon radical having an acyclic carbon atom attached to the nitrogen atoms. These dye polymers, with the dye molecules as part of the polymer backbone repeat unit, are unable to J-aggregate on the silver halide due to the relative orientation of the dye molecule units to each other. It is postulated that because of their orientation to each other as part of the polymer backbone, the dye molecules do not achieve the .pi. orbital overlap believed to be necessary for J-aggregation (see James, The Theory of the Photographic Process 4th, pp. 218-222, 1977).
J-aggregation of dye polymers has been achieved through ionic complexing of the dye molecule and the polymer (e.g., R. Bean, W. Shepard, R. Kay, & E. Walurck, J. Phys. Chem, 69 (12), 4368 (1965); however, these dye polymers have not been successfully used to spectrally sensitize silver halide. Because of competing ionic interactions and/or weak binding, the adsorption of these complexed dye polymers onto silver halide cannot be adequately controlled. Often, in the process of adding the complexed dyes to silver halide, the aggregates disassemble and behave like free dye.